1. The Field of the Invention
Semiconductor devices are generally manufactured from wafer-thin slices taken from a single crystal of a semiconductor composition, typically silicon, gallium arsenide, and other similar materials. The most common semiconductor devices are integrated circuits used as microprocessors and memory chips. An ingot of single crystal is obtained as a large boule from a molten bath of a crystallizable semiconductor material. A charge of the solid semiconductor material contained in a crucible is heated in a special furnace until the material melts. A rod having a seed crystal attached thereto, known as a crystal puller, is lowered into the molten bath and slowly withdrawn at a controlled rate that allows the melt to solidify and build up as a solidified single crystal at the end of the crystal puller. This single crystal, after cooling, is then sliced into wafers that are etched and further processed into high value semiconductor devices.
The present invention generally relates to furnace hardware formed from three-dimensional (3-D) carbon-carbon (C-C) composites, to methods for making such hardware, to a crystal pulling furnace apparatus incorporating such hardware, and to methods for manufacturing crystals using such hardware.
2. The State of the Art.
The crystal pulling furnace used in the manufacture of semiconductor materials contains predominantly polycrystalline graphite hardware; such a composition is conventionally chosen for the hardware because of its purity and ability to withstand the high temperatures of the crystal pulling process (approximately 1500.degree. C. for silicon). More particularly, the semiconductor material to be melted and crystallized (silicon being used generally herein as an example) in the crystal pulling furnace is contained in a quartz (silica glass) crucible, which in turn is supported and heated through a graphite crucible holder (also called a "susceptor"). The graphite crucible holder is typically composed of a bottom bowl section and a mating cylindrical top. The entire container assembly all sits on a pedestal. The quartz crucible and the graphite crucible holder are radiatively heated by a surrounding graphite resistive heating element; surrounding the heating element is a heat shield, then a layer of insulation, and finally the inner wall of the furnace.
During each run of the furnace, silicon is placed into the quartz crucible and heated. As the temperature increases, both the graphite and the quartz expand. Graphite has a higher coefficient of thermal expansion (COTE) than quartz, but the quartz tends to soften and thus conform to the overlying graphite crucible holder; the softened nature of the quartz lessens thermal stress imposed by the crucible and the holder on each other that would be due to differences in their coefficients of thermal expansion. However, after the pulling process is completed and the furnace starts to cool down, the quartz freezes in its expanded state and restrains the graphite from contracting, thus imposing a hoop tensile load on the wall of the crucible holder. To avoid these cool down stresses, conventional practice is to machine a vertical expansion slot into the wall of the crucible holder. Without this expansion slot, the graphite crucible holder would typically fail after one cycle of the furnace. The use of this expansion slot allows the graphite crucible holder to withstand the hoop tensile stresses for about five to 25 pulls, depending upon the type of graphite used and the diameter of the crucible. Nevertheless, the crucible holder and other parts of the furnace and support equipment, usually made from polycrystalline graphite, must each be replaced on a rather frequent periodic basis; typical replacement times are at least monthly for the crucible holder top, approximately monthly for the heater, and approximately annually for the pedestal.
Matsuo et al., in U.S. Pat. No. 5,207,992, describe a crystal pulling apparatus including a two-dimensional C-C composite crucible holder. Two-dimensional (2-D) composites are generally made by winding carbon fiber into a cylindrical geometry and thereafter densifying.
The manufacture of three-dimensional carbon-carbon composites is generally known, and a description of such can be found in P. S. Bruno et al., "Automatically Woven Three-Directional Composite Structures", SAMPE Quarterly, vol. 17, no. 4, Jul. 1986, pp. 10-17, the disclosure of which is incorporated herein by reference. Bruno et al. describe the production of both two-dimensional and three-dimensional composites for such applications as nozzles, cones, and other parts for rocket motors, aircraft and aerospace structures, and end fitting joints. Likewise, L. E. McAllister, in "Multidirectionally Reinforced Carbon/Graphite Matrix Composites," Engineered Materials Handbook, Vol. 1- Composites (Metals Park, Ohio: ASM International 1987) (the disclosure of which is incorporated herein by reference), describes the manufacture of multidimensional woven preforms and their densification for use as aerospace components, turbine engines, and biocompatible materials.
lnman et al., in U.S. Pat. No. 4,519,290 (the disclosure of which is incorporated herein by reference), describe 4-D architecture braided fiber preforms to annular refractory articles such as an integral monolithic throat section and an exit cone for a rocket motor nozzle.
Suto et al., in U.S. Pat. No. 4,975,262, describe 3-D woven fabrics of pitch-derived carbon fibers having bent parts of a small radius of curvature (e.g., 4 mm); the fabrics are useful as one component of fiber-composite-material and as a reinforcing materials in plastics, metals, cements, and ceramics.